Free chlorine (FC)
Also called: FC.
Free chlorine (FC) is the chlorine in pool water that is still available to sanitize, and it is the number a chlorine test reports as FC. Free chlorine has no single correct level: the minimum is about 7.5% of the pool's cyanuric acid (CYA) reading.
Why it matters
CYA binds most of your chlorine into a protected reserve, so only a small fraction is active at any moment. The more CYA in the water, the higher free chlorine has to be for the same killing power. Hold it too low for your CYA and algae grows even though the reading looks normal, which is the most common cause of a green pool with fine-looking chemistry. Read your target off the FC/CYA chart, and lean toward the top of the range in the peak of summer and after a pool party.
In numbers
At CYA 40, target free chlorine is 5–7 ppm and the minimum is 3. Below the minimum for your CYA, algae can grow even though the reading looks normal.